A method is known for extracting and regenerating rhenium and tungsten from rhenium-tungsten alloys by dissolving the alloy in hydrogen peroxide, heating the solution in a water bath and subjecting the resulting solution, containing HReO.sub.4, H.sub.2 WO.sub.5 and H.sub.2 O.sub.2, to boiling in order to decompose the excess hydrogen peroxide and to convert H.sub.2 WO.sub.5 into tungstic acid which forms a precipitate and is filtered through a paper filter or a glass crucible C-4. After rinsing twice the tungstic acid using hot water, the filtrate containing perrhenic acid is recovered and rhenium is separated in the form of potassium or ammonium perrhenate (see A. I. Lazerev, Zh. priklad. khim, 33, 468 (1960)).
In another modification of the method, the solution is heated to boiling, precipitated with potassium hydroxide and allowed to stay for 2 hours in a water-bath. Then it is cooled and the resulting precipitate of calcium tungstate is filtered and rinsed with a saturated solution of potassium hydroxide. The filtrate that contains Ca.sup.2+ and ReO.sub.4.sup.- is run through an ion exchange column filled with cationite KY-2 for the retention of Ca.sup.2+. To obtain full recovery of perrhenic acid, the column is scrubbed several times using water. The eluate solution together with the scrubbing waters is neutralized with potassium hydroxide or ammonia and is evaporated by infrared heating to dry crystals of potassium or ammonium perrhenate, respectively.
A disadvantage of this method is that the dissolution process is accompanied by intensive decomposition of hydrogen peroxide on the surface of the alloy. Consequently, the solution has to be replaced periodically by fresh amounts of peroxide. The large number of operations used in the method is time-consuming and labor-consuming and does not ensure complete separation of the extracted elements.
Another method for extracting rhenium for scraps of tungsten alloys treats the alloy with sodium nitrate at 500.degree.-600.degree. C., the water-soluble salts being extracted from the resulting melt using hot water (see Report dated Aug. 31, 1975 of Prof. Ortwin Bobleter, Institute of Radiochemistry and Applied Physical Chemistry, University of Insbruck, Austria). From the resulting solution that contains sodium perrhenate and sodium tungstate, rhenium is precipitated as potassium perrhenate by adding potassium chloride and tungstate remains in the solution.
A disadvantage of the method is the release of large amounts of nitrogen oxides which pollute the air. The method also requires working under strong ventilation and involves a considerable loss of rhenium.